Applied Surface Science 253 (2007) 6595–6598
www.elsevier.com/locate/apsusc
3D integration of microcomponents in a single glass chip by
femtosecond laser direct writing for biochemical analysis
Koji Sugioka *, Yasutaka Hanada, Katsumi Midorikawa
RIKEN—The Institute of Physical and Chemical Research, Laser Technology Laboratory, Wako, Saitama 351-0198, Japan
Available online 30 January 2007
Abstract
3D integration of microcomponents in a single glass chip by femtosecond laser direct writing followed by post annealing and successive wet
etching is described for application to biochemical analysis. Integration of microfluidics and microoptics realized some functional microdevices
like a m-fluidic dye laser and a biosensor. As one of practical applications, we demonstrate inspection of living microorganisms using the microchip
with 3D microfluidic structures fabricated by the present technique.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Femtosecond laser; 3D microfabrication; Laser direct writing; Microchip; Biochemical analysis
1. Introduction
In the last decade, it has been realized that miniaturization of
the ‘‘field’’ is urgently demanded in chemical reaction, biological
analysis and medical inspection, which emphasizes the use of
microcomponents three-dimensionally (3D) integrated in the
microchips. Examples of the miniaturized microchip devices are
the so-called Lab-on-a-chip devices and micro total analysis
system (m-TAS). By shrinking a roomful of laboratory
equipments and packing them into a palm-size chip, such
microchip devices are capable of performing chemical and
biological analyses with great reduction of reagent consumption,
waste production, analysis time and labor cost. It has been shown
that femtosecond (fs) laser is a promising tool for manufacture of
integrated microcomponents in glass due to its ability of internal
modification of transparent materials using multiphoton absorption. So far, a broad variety of photonic micrpcomponents, such
as waveguides, couplers, gratings, and Fresnel zone plates have
been fabricated inside the glass [1–3]. In addition, fs laser is also a
good tool for fabrication of microfluidic structures embedded in
the glass, like 3D microfluidic channels [4,5]. Recently, we
developed the technique that directly forms 3D hollow
microstructures with smooth internal surfaces inside glass by
* Corresponding author. Tel.: +81 48 467 9495; fax: +81 48 462 4682.
E-mail address: [email protected] (K. Sugioka).
0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2007.01.072
fs laser direct writing followed by post annealing and successive
wet etching [6–14]. This technique can fabricate both 3D
microfluidic and microoptic components in a single glass chip by
a single procedure, so that, for integration of each microcomponent, the cumbersome alignment process can be
eliminated and hereby the number of process steps can be
reduced.
In the present paper, fabrication of 3D hollow microstructures embedded in the glass is demonstrated by the fs laser
direct writing followed by the post annealing and the successive
wet etching. Microfluids and microoptics are integrated in a
single glass chip for some functional devices. The fabricated
3D hollow microstructure is also applied to inspect movement
of living microorganisms.
2. Experimental
Experiments described here were carried out by a
commercial fs laser workstation [6]. The laser wavelength,
pulse width and repetition rate were 775 nm, 140 5 fs and
1 kHz, respectively. To ensure a high beam quality, the output
laser beam of 6 mm in diameter was reduced to 3 mm by an
aperture in front of the focusing system. The focusing system
was a 20 microscope objective with a numerical aperture
(N.A.) of 0.46. The substrate used in this study for 3D
microstructuring is photostructurable glass that is commercially available under the trade name of Foturan from Schott
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Glass Corporation [15]. Foturan glass wafers of 2 mm thickness
were cut into 5 mm 10 mm coupons before use. Samples
under fabrication was translated by a PC controlled XYZ stage
at a resolution of 0.5 mm. The fabrication process was
displayed on the PC monitor by a charge-coupled device
(CCD). The typical irradiation condition of fs laser was a
fluence of 78 mJ/cm2 and a scanning speed of 510 mm/s. After
the fs laser exposure step, the sample was subjected to a
programmed post annealing. First, the temperature was raised
from room temperature to 500 8C at 5 8C/min and held at this
temperature for 1 h; then it was raised to 605 8C at 3 8C/min
and again held for another hour. After the sample was cooled
down to the room temperature, it was soaked in a 10% HF
solution with an ultrasonic bath at variable period of etching
time, depending on the size of the hollow structures. The laserexposed regions can be preferentially etched away with a
contrast ratio of ca. 50 in etching selectivity. Lastly the sample
was cleaned in distilled water and dried by a nitrogen gas flow.
The details of selective etching mechanism of photostructurable glass are described elsewhere [16–18].
3. Results and discussion
By using the present technique, we succeeded in fabricating
a variety of 3D microfluidic structures embedded in the
photostructurable glass such as Y-branched microchannel
structure [6], microfluidic structure with vertical configuration
[6], and microfluidics with a microvalve that can switch flow
direction of fluid [10]. As one example, Fig. 1 shows crossshape microchannels 200 mm embedded below the surface and
connected to five reservoirs at the surface. The microchannels
as long as 2.8 mm were successfully fabricated. To avoid the
over etching effect, the total time for the wet etching was
carefully chosen to be 45 min. Thus, the diameter of the
microchannels of 45 mm was achieved. Notably, from the
overview picture of the structure in Fig. 1, one can see that this
diameter was almost unchanged in the full length of the
fabricated channel, namely, the long channels were formed
almost without taper.
The present technique can be also applied for fabrication of
some microoptics embedded in the glass. The hollow
microplate acted as a micromirror due to total internal
reflection since the refractive index of the hollow microplate
whose inside is air is smaller than that of the surrounding glass
[8]. By the similar way, microbeam splitters and microoptical
circuits were realized [8]. Another microoptics fabricated by
the present technique was free standing optical fibers that can
guide the light beam in the glass microchip [14].
Since every microcomponent such as microchannels,
microreservoirs, microoptics, and optical fibers can be
fabricated by the same procedure, microfluidic and microoptical components can be easily integrated in a single glass
chip. To demonstrate it, we fabricated microfluidic dye lasers
which are useful as light sources for optical analyses such as
fluorescence detection or photoabsorption spectroscopy in the
Lab-on-a-chip devices since it can cover any wavelength in a
visible range by changing the kind of laser dye [11]. By threedimensionally stacking microfluidic components, we also
realized the micro twin lasers that emit an array of two laser
beams at different wavelengths by a single pumping [13]. Of
course, the microfluidic dye laser can be further integrated with
other microfluidics and microoptics for the Lab-on-a-chip
application.
Another integrated device we fabricated recently was a tiny
biosensor for photoabsorption measurement [19]. In this
device, the freestanding fibers were incorporated into a
microfluidic circuit, namely, two series of freestanding fibers
intercepted by a microreservoir was fabricated in the glass chip.
The fabricated device was applied for fluorescence detection of
a liquid sample. For the detection, the microreservoir was filled
with laser dye Rh640 and then the probe laser beam was
introduced into the one of fibers. The probe laser beam was
Fig. 1. Cross-shape microchannels 200 mm embedded below the surface and connected to five microreservoirs at the surface by fs laser direct writing followed by
post annealing and successive wet etching of photostructurable glass.
K. Sugioka et al. / Applied Surface Science 253 (2007) 6595–6598
6597
Fig. 2. Schematic illustrations of a microchip fabricated for the inspection of
Euglena.
guided by the fiber and then incident on the microreservoir
filled with the laser dye Rh640, so that bright fluorescence from
the dye was clearly observed at the microreservoir. A little part
of the fluorescence light was further guided by another
freestanding fiber. The guided fluorescence light would be
detected by a spectrometer.
More practical device fabricated by the present technique is
a microchip for inspection of microorganisms. To inspect
movement of Euglena’s flagellum is of great interest for
biologists due to application to biomotors and clarification of
the fertilization process. Currently, many biologists are trying
to observe microorganisms placed on a slide glass with a cover
glass by an optical microscope with high-speed camera.
However, the high numerical aperture objective lens used for
the observation limits the field of view (FOV) to a very narrow
region and also limits the depth of focus (DOE) to a very
shallow region, thereby making it difficult to capture images of
moving microorganisms. Consequently, it takes very long time
to take significant images. To shorten the observation time is
strongly demanded for biologists due to not only costeffectiveness and time-effectiveness but also due to limited
PC memory for taking movies using the high-speed camera. To
overcome these problems, we propose to use the microchip for
the observation of microorganisms. The microchip can scale
down the observation site, namely, it can three-dimensionally
encapsulate microorganisms in a limited area, so that it makes
much easier to capture the images of moving microorganisms.
Fig. 4. Picture of living Euglena taken using the microchip.
Fig. 2 shows schematic illustrations of a microchip
fabricated for the inspection of Euglena. For the observation,
Euglenas are introduced into one of reservoirs filled with water
and then Euglenas swim into the channel. At this moment, we
can see the movement of Euglena using a microscope from the
top of the glass surface. The most important factor to fabricate
the microchip is the distance between the glass surface and the
channel of 130–170 mm, which is determined by a working
distance of the objective lens for the observation. Additionally,
the etched glass surface must be smooth and also the top wall of
the channel must be flat and parallel to the glass surface for
taking clear images.
Fig. 3 shows optical microscope photographs of the
fabricated microchip. From the top view as shown in
Fig. 3(a), one can see that the reservoirs and channel with
demanded size are fabricated. Fig. 3(b) shows a cross-section of
the microchip when cutting the chip along the dashed line in
Fig. 3(a). The microchannel is embedded 150 mm below the
glass surface that satisfies the demand. The top of channel is flat
and parallel to the surface of the glass. Such a flat surface was
achieved by multiple scanning of the laser beam with lateral
Fig. 3. Optical microscope photographs of the fabricated microchip. (a) Top view, and (b) cross-section of the microchip when cutting the chip along the dashed line
in (a).
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shifts. In addition, post thermal treatment was carried out after
wet etching to smooth the etched surfaces [7].
Fig. 4 shows a picture of living Euglena taken using the
microchip. We also succeeded in taking a movie that revealed
that Euglena coils his flagellum around his body and rotates it
with high-speed to get a driving force when he moves straight.
The observation using the microchip has several advantages
over the conventional observation method: (1) analysis time can
be greatly reduced. For example, it takes only a few seconds to
take a one shot picture while more than 10 min even in lucky
case by the conventional method. Furthermore, it takes several
minutes to take a movie while very long time or even quite
difficult by the conventional method, (2) 3D observation is
possible, and (3) the living microorganisms can be kept very
fresh for long time since the microchannel three-dimensionally
confined in the glass can avoid vaporization of water.
4. Conclusions
We have demonstrated 3D integration of microcomponents
in photostructurable glass by femtosecond laser direct writing
followed by the post annealing and successive wet etching for
biochemical analysis. A single procedure forming 3D hollow
microstructures can fabricate various microcomponents such as
microfluidic channels, micromechanical parts, microoptics, and
optical fibers, resulting in elimination of cumbersome
alignment process for integration of each microcomponent
and thereby reduction of process steps. Integration of
microfluidic and microoptical components realized m-fluidic
dye lasers and biosensors. More practical device fabricated by
the present technique was a microchip for inspection of living
microorganisms that has several advantages over the conven-
tional inspection method. Finally, we conclude that the
technique presented here is very promising for manufacture
of highly integrated Lab-on-a-chip devices.
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